Epigenetic modifications can impact genetic variation but, when dysregulated, can also contribute to the development of various diseases (Portela, A. and M. Esteller, Epigenetic modifications and human disease. Nat Biotechnol, 2010. 28(10): p. 1057-68; Lund, A. H. and M. van Lohuizen, Epigenetics and cancer. Genes Dev, 2004. 18(19): p. 2315-35). Recently, in depth cancer genomics studies have discovered many epigenetic regulatory genes are often mutated or their own expression is abnormal in a variety of cancers (Dawson, M. A. and T. Kouzarides, Cancer epigenetics: from mechanism to therapy. Cell, 2012. 150(1): p. 12-27; Waldmann, T. and R. Schneider, Targeting histone modifications—epigenetics in cancer. Curr Opin Cell Biol, 2013. 25(2): p. 184-9; Shen, H. and P. W. Laird, Interplay between the cancer genome and epigenome. Cell, 2013. 153(1): p. 38-55). This implies epigenetic regulators function as cancer drivers or are permissive for tumorigenesis or disease progression. Therefore, deregulated epigenetic regulators are attractive therapeutic targets.
One particular enzyme which is associated with human diseases is lysine specific demethylase-1 (LSD1), the first discovered histone demethylase (Shi, Y., et al., Histone demethylation mediated by the nuclear amine oxidase homolog LSD1. Cell, 2004. 119(7): p. 941-53). It consists of three major domains: the N-terminal SWIRM which functions in nucleosome targeting, the tower domain which is involved in protein-protein interaction, such as transcriptional co-repressor, co-repressor of RE1-silencing transcription factor (CoREST), and lastly the C terminal catalytic domain whose sequence and structure share homology with the flavin adenine dinucleotide (FAD)-dependent monoamine oxidases (i.e., MAO-A and MAO-B) (Forneris, F., et al., Structural basis of LSD1-CoREST selectivity in histone H3 recognition. J Biol Chem, 2007. 282(28): p. 20070-4; Anand, R. and R. Marmorstein, Structure and mechanism of lysine-specific demethylase enzymes. J Biol Chem, 2007. 282(49): p. 35425-9; Stavropoulos, P., G. Blobel, and A. Hoelz, Crystal structure and mechanism of human lysine-specific demethylase-1. Nat Struct Mol Biol, 2006. 13(7): p. 626-32; Chen, Y., et al., Crystal structure of human histone lysine-specific demethylase 1 (LSD1). Proc Natl Acad Sci USA, 2006. 103(38): p. 13956-61). LSD1 also shares a fair degree of homology with another lysine specific demethylase (LSD2) (Karytinos, A., et al., A novel mammalian flavin-dependent histone demethylase. J Biol Chem, 2009. 284(26): p. 17775-82). Although the biochemical mechanism of action is conserved in two isoforms, the substrate specificities are thought to be distinct with relatively small overlap. The enzymatic reactions of LSD1 and LSD2 are dependent on the redox process of FAD and the requirement of a protonated nitrogen in the methylated lysine is thought to limit the activity of LSD1/2 to mono- and di-methylated at the position of 4 or 9 of histone 3 (H3K4 or H3K9). These mechanisms make LSD1/2 distinct from other histone demethylase families (i.e. Jumonji domain containing family) that can demethylate mono-, di-, and tri-methylated lysines through alpha-ketoglutarate dependent reactions (Kooistra, S. M. and K. Helie, Molecular mechanisms and potential functions of histone demethylases. Nat Rev Mol Cell Biol, 2012. 13(5): p. 297-311; Mosammaparast, N. and Y. Shi, Reversal of histone methylation: biochemical and molecular mechanisms of histone demethylases. Annu Rev Biochem, 2010. 79: p. 155-79).
Methylated histone marks on K3K4 and H3K9 are generally coupled with transcriptional activation and repression, respectively. As part of corepressor complexes (e.g., CoREST), LSD1 has been reported to demethylate H3K4 and repress transcription, whereas LSD1, in nuclear hormone receptor complex (e.g., androgen receptor), may demethylate H3K9 to activate gene expression (Metzger, E., et al., LSD1 demethylates repressive histone marks to promote androgen-receptor-dependent transcription. Nature, 2005. 437(7057): p. 436-9; Kahl, P., et al., Androgen receptor coactivators lysine-specific histone demethylase 1 and four and a half LIM domain protein 2 predict risk of prostate cancer recurrence. Cancer Res, 2006. 66(23): p. 11341-7). This suggests the substrate specificity of LSD1 can be determined by associated factors, thereby regulating alternative gene expressions in a context dependent manner. In addition to histone proteins, LSD1 may demethylate non-histone proteins. These include p53 (Huang, J., et al., p53 is regulated by the lysine demethylase LSD1. Nature, 2007. 449(7158): p. 105-8), E2F (Kontaki, H. and I. Talianidis, Lysine methylation regulates E2F1-induced cell death. Mol Cell, 2010. 39(1): p. 152-60), STAT3 (Yang, J., et al., Reversible methylation of promoter-bound STAT3 by histone-modifying enzymes. Proc Natl Acad Sci USA, 2010. 107(50): p. 21499-504), Tat (Sakane, N., et al., Activation of HIV transcription by the viral Tat protein requires a demethylation step mediated by lysine-specific demethylase 1 (LSD1/KDM1). PLoS Pathog, 2011. 7(8): p. e1002184), and myosin phosphatase target subunit 1 (MYPT1) (Cho, H. S., et al., Demethylation of RB regulator MYPT1 by histone demethylase LSD1 promotes cell cycle progression in cancer cells. Cancer Res, 2011. 71(3): p. 655-60). The lists of non-histone substrates are growing with technical advances in functional proteomics studies. These suggest additional oncogenic roles of LSD1 beyond in regulating chromatin remodeling. LSD1 also associates with other epigenetic regulators, such as DNA methyltransferase 1 (DNMT1) (Wang, J., et al., The lysine demethylase LSD1 (KDM1) is required for maintenance of global DNA methylation. Nat Genet, 2009. 41(1): p. 125-9) and histone deacetylases (HDACs) complexes (Hakimi, M. A., et al., A core-BRAF35 complex containing histone deacetylase mediates repression of neuronal-specific genes. Proc Natl Acad Sci USA, 2002. 99(11): p. 7420-5; Lee, M. G., et al., Functional interplay between histone demethylase and deacetylase enzymes. Mol Cell Biol, 2006. 26(17): p. 6395-402; You, A., et al., CoREST is an integral component of the CoREST-human histone deacetylase complex. Proc Natl Acad Sci USA, 2001. 98(4): p. 1454-8). These associations augment the activities of DNMT or HDACs. LSD1 inhibitors may therefore potentiate the effects of HDAC or DNMT inhibitors. Indeed, preclinical studies have shown such potential already (Singh, M. M., et al., Inhibition of LSD1 sensitizes glioblastoma cells to histone deacetylase inhibitors. Neuro Oncol, 2011. 13(8): p. 894-903; Han, H., et al., Synergistic re-activation of epigenetically silenced genes by combinatorial inhibition of DNMTs and LSD1 in cancer cells. PLoS One, 2013. 8(9): p. e75136).
LSD1 has been reported to contribute to a variety of biological processes, including cell proliferation, epithelial-mesenchymal transition (EMT), and stem cell biology (both embryonic stem cells and cancer stem cells) or self-renewal and cellular transformation of somatic cells (Chen, Y., et al., Lysine-specific histone demethylase 1 (LSD1): A potential molecular target for tumor therapy. Crit Rev Eukaryot Gene Expr, 2012. 22(1): p. 53-9; Sun, G., et al., Histone demethylase LSD1 regulates neural stem cell proliferation. Mol Cell Biol, 2010. 30(8): p. 1997-2005; Adamo, A., M. J. Barrero, and J. C. Izpisua Belmonte, LSD1 and pluripotency: a new player in the network. Cell Cycle, 2011. 10(19): p. 3215-6; Adamo, A., et al., LSD1 regulates the balance between self-renewal and differentiation in human embryonic stem cells. Nat Cell Biol, 2011. 13(6): p. 652-9). In particular, cancer stem cells or cancer initiating cells have some pluripotent stem cell properties that contribute the heterogeneity of cancer cells. This feature may render cancer cells more resistant to conventional therapies, such as chemotherapy or radiotherapy, and then develop recurrence after treatment (Clevers, H., The cancer stem cell: premises, promises and challenges. Nat Med, 2011. 17(3): p. 313-9; Beck, B. and C. Blanpain, Unravelling cancer stem cell potential. Nat Rev Cancer, 2013. 13(10): p. 727-38). LSD1 was reported to maintain an undifferentiated tumor initiating or cancer stem cell phenotype in a spectrum of cancers (Zhang, X., et al., Pluripotent Stem Cell Protein Sox2 Confers Sensitivity to LSD1 Inhibition in Cancer Cells. Cell Rep, 2013. 5(2): p. 445-57; Wang, J., et al., Novel histone demethylase LSD1 inhibitors selectively target cancer cells with pluripotent stem cell properties. Cancer Res, 2011. 71(23): p. 7238-49). Acute myeloid leukemias (AMLs) are an example of neoplastic cells that retain some of their less differentiated stem cell like phenotype or leukemia stem cell (LSC) potential. Analysis of AML cells including gene expression arrays and chromatin immunoprecipitation with next generation sequencing (ChIP-Seq) revealed that LSD1 may regulate a subset of genes involved in multiple oncogenic programs to maintain LSC (Harris, W. J., et al., The histone demethylase KDM1A sustains the oncogenic potential of MLL-AF9 leukemia stem cells. Cancer Cell, 2012. 21(4): p. 473-87; Schenk, T., et al., Inhibition of the LSD1 (KDM1A) demethylase reactivates the all-trans-retinoic acid differentiation pathway in acute myeloid leukemia. Nat Med, 2012. 18(4): p. 605-11). These findings suggest potential therapeutic benefit of LSD1 inhibitors targeting cancers having stem cell properties, such as AMLs.
Overexpression of LSD1 is frequently observed in many types of cancers, including bladder cancer, NSCLC, breast carcinomas, ovary cancer, glioma, colorectal cancer, sarcoma including chondrosarcoma, Ewing's sarcoma, osteosarcoma, and rhabdomyosarcoma, neuroblastoma, prostate cancer, esophageal squamous cell carcinoma, and papillary thyroid carcinoma. Notably, studies found over-expression of LSD1 was significantly associated with clinically aggressive cancers, for example, recurrent prostate cancer, NSCLC, glioma, breast, colon cancer, ovary cancer, esophageal squamous cell carcinoma, and neuroblastoma. In these studies, either knockdown of LSD1 expression or treatment with small molecular inhibitors of LSD1 resulted in decreased cancer cell proliferation and/or induction of apoptosis. See, e.g., Hayami, S., et al., Overexpression of LSD1 contributes to human carcinogenesis through chromatin regulation in various cancers. Int J Cancer, 2011. 128(3): p. 574-86; Lv, T., et al., Over-expression of LSD1 promotes proliferation, migration and invasion in non-small cell lung cancer. PLoS One, 2012. 7(4): p. e35065; Serce, N., et al., Elevated expression of LSD1 (Lysine-specific demethylase 1) during tumour progression from pre-invasive to invasive ductal carcinoma of the breast. BMC Clin Pathol, 2012. 12: p. 13; Lim, S., et al., Lysine-specific demethylase 1 (LSD1) is highly expressed in ER-negative breast cancers and a biomarker predicting aggressive biology. Carcinogenesis, 2010. 31(3): p. 512-20; Konovalov, S. and I. Garcia-Bassets, Analysis of the levels of lysine-specific demethylase 1 (LSD1) mRNA in human ovarian tumors and the effects of chemical LSD1 inhibitors in ovarian cancer cell lines. J Ovarian Res, 2013. 6(1): p. 75; Sareddy, G. R., et al., KDM1 is a novel therapeutic target for the treatment of gliomas. Oncotarget, 2013. 4(1): p. 18-28; Ding, J., et al., LSD1-mediated epigenetic modification contributes to proliferation and metastasis of colon cancer. Br J Cancer, 2013. 109(4): p. 994-1003; Bennani-Baiti, I. M., et al., Lysine-specific demethylase 1 (LSD1/KDM1A/AOF2/BHC110) is expressed and is an epigenetic drug target in chondrosarcoma, Ewing's sarcoma, osteosarcoma, and rhabdomyosarcoma. Hum Pathol, 2012. 43(8): p. 1300-7; Schulte, J. H., et al., Lysine-specific demethylase 1 is strongly expressed in poorly differentiated neuroblastoma: implications for therapy. Cancer Res, 2009. 69(5): p. 2065-71; Crea, F., et al., The emerging role of histone lysine demethylases in prostate cancer. Mol Cancer, 2012. 11: p. 52; Suikki, H. E., et al., Genetic alterations and changes in expression of histone demethylases in prostate cancer. Prostate, 2010. 70(8): p. 889-98; Yu, Y., et al., High expression of lysine-specific demethylase 1 correlates with poor prognosis of patients with esophageal squamous cell carcinoma. Biochem Biophys Res Commun, 2013. 437(2): p. 192-8; Kong, L., et al., Immunohistochemical expression of RBP2 and LSD1 in papillary thyroid carcinoma. Rom J Morphol Embryol, 2013. 54(3): p. 499-503.
Recently, the induction of CD86 expression by inhibiting LSD1 activity was reported (Lynch, J. T., et al., CD86 expression as a surrogate cellular biomarker for pharmacological inhibition of the histone demethylase lysine-specific demethylase 1. Anal Biochem, 2013. 442(1): p. 104-6). CD86 expression is a marker of maturation of dendritic cells (DCs) which are involved in antitumor immune response. Notably, CD86 functions as a co-stimulatory factor to activate T cell proliferation (Greaves, P. and J. G. Gribben, The role of B7 family molecules in hematologic malignancy. Blood, 2013. 121(5): p. 734-44; Chen, L. and D. B. Flies, Molecular mechanisms of T cell co-stimulation and co-inhibition. Nat Rev Immunol, 2013. 13(4): p. 227-42).
In addition to playing a role in cancer, LSD1 activity has also been associated with viral pathogenesis. Particularly, LSD1 activity appears to be linked with viral replications and expressions of viral genes. For example, LSD1 functions as a co-activator to induce gene expression from the viral immediate early genes of various type of herpes virus including herpes simplex virus (HSV), varicella zoster virus (VZV), and β-herpesvirus human cytomegalovirus (Liang, Y., et al., Targeting the JMJD2 histone demethylases to epigenetically control herpesvirus infection and reactivation from latency. Sci Transl Med, 2013. 5(167): p. 167ra5; Liang, Y., et al., Inhibition of the histone demethylase LSD1 blocks alpha-herpesvirus lytic replication and reactivation from latency. Nat Med, 2009. 15(11): p. 1312-7). In this setting, a LSD1 inhibitor showed antiviral activity by blocking viral replication and altering virus associated gene expression.
Recent studies have also shown that the inhibition of LSD1 by either genetic depletion or pharmacological intervention increased fetal globin gene expression in erythroid cells (Shi, L., et al., Lysine-specific demethylase 1 is a therapeutic target for fetal hemoglobin induction. Nat Med, 2013. 19(3): p. 291-4; Xu, J., et al., Corepressor-dependent silencing of fetal hemoglobin expression by BCL11A. Proc Natl Acad Sci USA, 2013. 110(16): p. 6518-23). Inducing fetal globin gene would be potentially therapeutically beneficial for the disease of β-globinopathies, including β-thalassemia and sickle cell disease where the production of normal β-globin, a component of adult hemoglobin, is impaired (Sankaran, V. G. and S. H. Orkin, The switch from fetal to adult hemoglobin. Cold Spring Harb Perspect Med, 2013. 3(1): p. a011643; Bauer, D. E., S. C. Kamran, and S. H. Orkin, Reawakening fetal hemoglobin: prospects for new therapies for the beta-globin disorders. Blood, 2012. 120(15): p. 2945-53). Moreover, LSD1 inhibition may potentiate other clinically used therapies, such as hydroxyurea or azacitidine. These agents may act, at least in part, by increasing γ-globin gene expression through different mechanisms.
In summary, LSD1 contributes to tumor development by altering epigenetic marks on histones and non-histone proteins. Accumulating data have validated that either genetic depletion or pharmacological intervention of LSD1 normalizes altered gene expressions, thereby inducing differentiation programs into mature cell types, decreasing cell proliferation, and promoting apoptosis in cancer cells. Therefore, LSD1 inhibitors alone or in combination with established therapeutic drugs would be effective to treat the diseases associated with LSD1 activity.